An imager detects light and creates a digital image of a scene based on that detected light. The image contains a fixed number of rows and columns of pixels where each pixel maps to a different field-of-view within the scene. Electronic imagers typically make use of photodetectors to convert light into electrical signals. Each photodetector is located at a different position on the focal plane and usually corresponds to a single pixel or a component of a pixel in the image. Electronic imagers can typically be classified as one of two types: a passive-illumination imager or an active-illumination imager. A passive-illumination imager collects ambient light such as sunlight reflected by objects in a scene, whereas an active-illumination imager illuminates the scene and collects reflected light generated by the active-illumination imager system itself.
A narrowband imager collects light within a limited wavelength range. This is in contrast to a traditional camera which detects light across the entire visible spectrum or into three different wide, RGB color bands, each of which may be 100 nm or wider. Narrowband imagers are harder to develop than traditional cameras due to the characteristics of the optical filters on which they rely. Optical filters serve to prevent some portion of the electromagnetic spectrum from reaching the photodetectors. Most narrowband filters rely on thin-film interference effects to selectively transmit or reflect light (such filters are often referred to as dielectric mirrors or Bragg mirrors). The spectral transmissivity of the narrowband filter depends on the number, thicknesses, ordering, and indices of refraction of the constituent layers forming the filter. The spectral transmissivity of the filter also depends upon the angle of incidence of the light upon the narrowband filter.
Current narrowband imagers have either a small field-of-view or are limited in their ability to filter wavelength bands narrower than around 50 nm. Optical filters are sensitive to the angle of incident light making it difficult to achieve a narrow range of wavelengths. For example, an optical filter may accept perpendicular light with wavelength at 940-945 nm and slightly oblique light at a wavelength of 930-935 nm. Since most photodetectors in a traditional camera have a large range of angles of light incident upon them, simply placing an optical filter in front of them would not actually achieve narrowband filtering. Constricting the angle of light incident upon the photodetector usually requires using a lens with a longer focal length, which constricts the field-of-view of the camera.
Imagers with a wide field-of-view have difficulty in generating uniformly clear visual images and in making uniform measurements across a scene. For example, the pixels at the center of the image may appear brighter or represent a different wavelength of light compared to the pixels at the scene extremities. A wide field-of-view is desirable for some applications because it provides better situational awareness. For example, a camera-based automotive safety system meant to detect pedestrians around a vehicle might require monitoring in a 360 degree field-of-view around the vehicle. Fewer wide field-of-view sensors are required to do the same job (i.e., generate images of the full 360 degree field-of-view) as many narrow field of view sensors, thereby decreasing the system cost.
Narrowband imagers have many applications including geographic mapping, astronomy and in LIDAR (Light Detection and Ranging). Narrowband imagers can detect characteristic light wavelengths such as those generated by plants with chlorophyll or by elements within stars. Narrowband imagers can be used, for example, to determine vegetation health or to discover oil deposits. Optical receiver systems, such as LIDAR, can be used for object detection and ranging. LIDAR systems measure the distance to a target or objects in a landscape, by irradiating a target or landscape with light, using pulses from a laser, and measuring the time it takes photons to travel to the target or landscape and return after reflection to a narrowband imager. Other LIDAR techniques, such as photo-demodulation, coherent LIDAR, and range-gated LIDAR, also rely on the transmission and reflection of photons, though they may not directly measure the time-of-flight of pulses of laser light. For many LIDAR applications, it is beneficial for physical sizes of transmitters and receivers to small and compact, and at the same time relatively low in cost. For applications where objects must be sensed with accuracy at long distances, it is beneficial to increase or maximize the number of photons emitted by the transmitter and reflected back toward the receiver while keeping laser energy emissions within mandated safety limits.
Micro-optical systems are systems that include miniaturized, optical components that are typically between a few micrometers and a millimeter in size. Micro-optical receivers arrayed adjacent to each other are susceptible to crosstalk. Stray light caused by roughness of optical surfaces, imperfections in transparent media, back reflections, etc., may be generated at various features within the receiver channel or external to receiver channel. When multiple receiver channels are arrayed adjacent to one another, this stray light in one receiver channel may be absorbed by a photosensor in another channel, thereby contaminating the timing, phase, or other information inherent to photons. Minimizing crosstalk is especially important in active-illumination systems. Light reflected from a nearby retro-reflector (e.g. a license plate) may be thousands or millions of time more intense than light reflected from a distant, dark, lambertian surface (e.g. black cotton clothing). Thus, the stray light photons from a retro-reflector could vastly outnumber photons reflected from other surfaces in nearby photosensors if crosstalk is not minimized. This can result in the inability of a LIDAR system to detect dark objects that occupy fields of view near the field of view occupied by a retro-reflector.